Hydraulic circuit arrangement with recovery of energy

ABSTRACT

The invention relates to a hydraulic circuit arrangement having a first pump which is driven by an electric motor and is assigned to a lifting cylinder, wherein, in order to recover energy, the electric motor can be operated as a generator and the first pump can be operated as a hydraulic motor in order to recover potential energy of the lifting cylinder, and having a second pump for supplying at least one further load. The electric motor, the first pump and the second pump are embodied as a motor/double pump assembly, wherein in the lowering mode of the lifting cylinder the first pump acts as a hydraulic motor with reversal of the rotational direction while the second pump always operates as a pump with a constant delivery direction. Arranged between the first pump and the lifting cylinder is a priority valve which, depending on requirements, distributes the volume flow which flows in from the first pump to the lifting cylinder on the one hand, and to the at least one further load on the other, and the volume flow which flows in from the lifting cylinder in the lowering mode accordingly to the at least one load and the first pump.

BACKGROUND OF THE INVENTION

The invention relates to a hydraulic circuit arrangement having a firstpump which is driven by an electric motor and is assigned to a liftingcylinder, wherein, in order to recover energy, the electric motor can beoperated as a generator and the first pump can be operated as ahydraulic motor in order to recover potential energy of the liftingcylinder, and having a second pump supplying at least one further loadaccording to the preamble of Claim 1.

Such a circuit arrangement for battery-driven industrial trucks is knownfrom DE 43 17 782 C2. In said document, a direct current or asynchronouselectric motor drives a pump P which makes available the hydraulicpressure for the lifting cylinder. When the load is lowered, the circuitarrangement clears a path on which the fluid can flow back virtuallywithout loss of pressure from the lifting cylinder to the pump P. Inthis context, the potential energy which is stored in the liftingcylinder is recovered by virtue of the fact that the pump then operatesas a hydraulic motor and the electric motor then operates as a generatorG for recharging the battery. The particular advantages lie in the factthat the expenditure is relatively low and no pressure losses due to theprinciple occur.

In the known arrangement, it also possible to connect other loads. Forthis purpose, a separate circuit with a further pump and separate drivemotor is provided. Furthermore, additional functions can be provided bythe first-mentioned working circuit. However, the lowering function mustthen be respectively interrupted for this purpose.

DE 299 11 686 U1 discloses a further electrohydraulic lifting modulewhich can be operated in an energy-saving mode. In this context, asingle pump supplies both main loads and secondary loads which can beconnected into the circuit via an electromagnetically actuated valve. Aseparate return line is provided for the lowering process of the liftingcylinder, and when said return line is opened it connects the liftingcylinder to the intake side of the pump, wherein a non-return valveprevents the direct emptying into the reservoir. The working line,otherwise provided for the lifting mode, of the pump then functions as areturn line and is connected to the tank via an electromagnetic valve.

SUMMARY OF THE INVENTION

The invention is based on the object of specifying an improved, morecost-effective circuit for recovering energy from a lifting cylinderwhile simultaneously operating with other loads.

According to the invention, this is achieved in a hydraulic circuitarrangement according to the preamble of Claim 1 by virtue of the factthat the electric motor, the first pump and the second pump are embodiedas a motor/double pump assembly, wherein in the lowering mode of thelifting cylinder the first pump acts as a hydraulic motor with reversalof the rotational direction while the second pump always operates as apump with a constant delivery direction, and wherein provided betweenthe first pump and the lifting cylinder is a priority valve which,depending on requirements, distributes the volume flow which flows infrom the first pump to the lifting cylinder on the one hand, and to theat least one further load on the other, and the volume flow which flowsin from the lifting cylinder in the lowering mode accordingly to the atleast one load and the first pump.

This solution reduces quite considerably the complexity of known circuitarrangements in terms of valve technology for supplying a plurality ofloads.

As preference, arranged in the working line of the first pump betweenthe priority valve and the lifting cylinder is an electricallycontrollable lifting valve which is partially opened during simultaneousoperation of the lifting cylinder and of the at least one further load,and in its two limiting positions switches the working line of the firstpump to the closed and open positions, wherein when the lifting cylinderand the at least one further load are not operating simultaneously, thelifting valve opens fully. The lifting valve ensures, together with thepriority valve, that excess volume flow of the first pump can flow tothe further loads, wherein it is added to the volume flow of the secondpump. The second pump can therefore be given relatively small dimensionsbecause it is assisted by the first pump.

It is particularly advantageous if the priority valve is embodied insuch a way that it is controlled by the pressure difference which occursat the lifting valve, wherein in a first extreme position of thepriority valve, the first pump is connected, on the one hand, to thelifting valve via a throttle and, on the other hand, to the at least onefurther load, while in a second opposed extreme position of the priorityvalve, the working line which comes from the first pump is switched tothe open position and at the same time is connected to the at least onefurther load, and the priority valve also has a central position inwhich the working line of the first pump is switched to the openposition and the line to the at least one further load is closed.

Preferably provided in the line leading from the priority valve to theat least one further load is a non-return valve which then opens if inthe working line leading from the first pump to the lifting cylinderthere is excess pressure above the pressure generated by the secondpump.

The supply of further loads can advantageously be carried out by aconstant flow system which is of particularly simple design and issufficient for a large number of practical applications. However, aload-sensing system which permits better pressure adaptation and volumeadaptation to the instantaneous requirements of the individual loads canpreferably also be provided for this purpose.

In a further advantageous embodiment of the invention, an electricallycontrollable load-lowering valve is provided, and in the lowering modesaid load-lowering valve diverts the volume flow from the liftingcylinder to the reservoir without the recovery of energy and switches inthe cases in which the recovery of energy is to be dispensed with. Sucha case can occur, for example, when a small lifting load is lowered andthere is simultaneous operation of further loads. If, in this context,only a small amount of energy is available for recovering, it is,according to circumstances, preferable to divert the volume flow intothe reservoir. Such situations can preferably be detected by means ofpressure sensors which are provided in the respective working lines ofthe first and the second pumps and are used to actuate the load-loweringvalve.

Further features and advantages of the invention emerge from thefollowing description of the figures:

BRIEF DESCRIPTION OF THE FIGURES

In said figures:

FIG. 1: shows a first exemplary embodiment of the invention in which thefurther loads are supplied by a load-sensing system.

FIG. 2: shows the exemplary embodiment from FIG. 1 with a load-loweringvalve for optionally bypassing the recovery of energy, and

FIG. 3: shows a further exemplary embodiment in which the additionalloads are supplied by a constant flow system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a first exemplary embodiment of the invention. Themotor/double pump assembly MDP comprises an electric motor M/G whichdrives a first pump P1 and a second pump P2 which has smallerdimensions, wherein the electric motor M/G and the pumps P1, P2 arearranged on a common shaft. The first pump P1 supplies the hydraulicpressure for a lifting cylinder HZ, and the second pump P2 suppliesfurther loads Z1, Z2.

When the load is lowered, potential energy of the lifting cylinder HZ isrecovered, wherein the first pump P1 then functions as a hydraulic motorwith reversal of the rotational direction, and the electric motor M/Gfunctions as a generator for recharging an accumulator (notillustrated). The second pump P2 always operates as a pump with aconstant delivery direction irrespective of the rotational direction.

The first pump P1 is port-controlled, i.e. its expeller spaces arealternately connected to the high pressure end and to the low pressureend via a valve plate. If the expeller space is connected to lowpressure in the intake stroke, that is to say when there is an increasein volume, the unit then acts as a pump, and if it is connected to thehigh pressure when there is an increase in volume, the unit thenoperates as a motor. When there is a changeover from the lifting modeinto the lowering mode of the lifting cylinder HZ, the unit P1 whichfirst operates as a pump changes its rotational direction withessentially unchanged pressure conditions, and therefore changessimultaneously into the motor mode.

The pump P2 is controlled instead of via a valve plate by virtue of thefact that each expeller space is assigned a non-return valve via whichthe connection to the high pressure and to the low pressure is made.Irrespective of the rotational direction, such a unit which iscontrolled via a non-return valve will thus always suck out of the lowpressure when there is an increase in volume and feed into the highpressure when there is a decrease in volume. The pump P2 cannot operateas a motor. This dependence on the rotational direction has anadvantageous effect on the present invention because the basic supply ofthe further loads Z1, Z2 is ensured irrespective of the lifting orlowering of the lifting cylinder.

Furthermore, provided in the working line from the pump P1 to thelifting cylinder HZ is a priority valve PRV which, depending onrequirements, distributes the volume flow which flows in from the firstpump P1 to the lifting cylinder HZ on the one hand, and to the furtherloads Z1, Z2 on the other, so that the pump P2 is correspondinglyassisted. This also occurs in the same way in the lowering mode in whichthe priority valve distributes the volume flow which is now flowing infrom the lifting cylinder HZ to the first pump P1, and, depending onrequirements, also to the further loads Z1, Z2.

Provided between the priority valve PRV and the lifting cylinder HZ is alifting valve HV which is embodied as an electrically actuated constantvalve and can be adjusted between the limiting positions of “closed” and“fully opened”.

The pressure difference which occurs across the lifting valve HV isapplied to the priority valve PRV. If no additional loads but ratheronly the lifting cylinder is to be activated, the lifting valve HV opensentirely so that there is no pressure drop across the control edge. Thespeed of the lifting cylinder HZ is then controlled by means of therotational speed of the pump P1, and the priority valve PRV is notdeflected from its central position in which the connection to thelifting cylinder HZ is fully opened and the branch to the further loadsis interrupted. In this situation, the pump P2, which rotates at thesame rotational speed, also delivers. The excess volume flow is divertedto the reservoir via the pressure balance DWP. This causes a certainloss of energy which, however, can be kept small if the pump P2 is givencorrespondingly small dimensions.

If only the further loads Z1, Z2 are activated, the lifting valveremains closed, as a result of which a pressure which pushes thepriority valve into the left-hand position in the drawing builds up. Inthis position, the working line of the pump P1 is connected via thenon-return valve RÜV to that of the pump P2 so that the volume flow ofpump P1 is added to that of the pump P2. The further loads Z1, Z2 are asa result supplied as if by a single pump. In this case, after there isno excess volume flow which would have to flow off via the pressurebalance DWP, no energy losses due to the principle occur in this modeeither.

If the lifting cylinder and further loads are activated simultaneously,the lifting valve is opened only partially so that an appreciablepressure drop occurs across its control edge, said pressure drop pushingthe priority valve PRV into the right-hand position in the drawing inwhich the pump P1 is connected both to the lifting cylinder and to theadditional loads Z1, Z2 via the non-return valve RÜV, and the pump P2 istherefore assisted.

The working lines of the pumps P1 and P2 are protected with respect tothe reservoir T by overpressure valves in a known way. Furthermore, forsafety reasons a load-holding valve LHV, which prevents the load fromsagging after the ending of the lifting process or prevents excessivelyfast lowering of the load, is inserted upstream of the lifting cylinderHZ.

The further loads Z1, Z2 can operate in various ways. In the exemplaryembodiment in FIG. 1, a load-sensing system LSS is provided for thispurpose. The division of the volume flow between the individual loadsZ1, Z2 with their different pressure levels occurs in a known fashionwith individual pressure balances DW1, DW2, downstream of which thedirectional control valves V1, V2 are connected. The changeover valve WVconnects the maximum pressure to the inlet pressure balance DWP wherethis pressure is compared with the system pressure which is present inthe working line of the pump P2.

During the operation of the hydraulic circuit according to theinvention, the following four cases can now be differentiated:

-   -   a) Lifting, lifting cylinder pressure higher than at the further        loads: in this case the pressure difference at the priority        valve PRV and at the individual pressure balances DW1, DW2 of        the further loads Z1, Z2 is throttled away. The power loss which        occurs grows with the speed of these loads.    -   b) Lifting, lifting cylinder pressure less than at the further        loads: in this case, the pressure difference is throttled away        only at the priority valve PRV. The power loss which occurs        grows with the speed of the lifting cylinder.    -   c) Lowering, lifting cylinder pressure higher than at the        further loads: in this case the pressure difference is throttled        away only at the individual pressure balances DW1, DW2 of the        further loads Z1, Z2. The power loss which occurs grows with the        speed of these loads. No pressure drop occurs in the priority        valve.    -   d) Lowering, lifting cylinder pressure lower than at the further        loads: in this case the non-return valve RÜV does not open. The        speed of the lifting cylinder is defined unambiguously by the        rotational speed of the pump P1. For the further loads this can        mean undersupply if the second pump P2 is given such small        dimensions that it is reliant on assistance from the first pump        P2. However, the function of the circuit, specifically the        independent operation of the lifting cylinder and further loads,        is basically still provided despite a certain reduction in        comfort.

In the last-mentioned case, in which only a small amount of energy isavailable in any case, it is possible, if appropriate, to dispense withthe recovery in the lowering mode by virtue of the fact that the volumeflow is diverted via a flow-regulating valve. This possibility isillustrated in FIG. 2. Apart from the load-lowering valve LSV and twopressure sensors D1, D2, this circuit is identical to that in FIG. 1.The reference symbols for the same circuit parts were retained. Thepreviously mentioned operating state d), in which in the lowering modethe lifting cylinder pressure is lower than at the further loads Z1, Z2,is detected with the pressure sensors D1, D2. If appropriate, the volumeflow is diverted from the lifting cylinder to the reservoir T via thecorrespondingly actuated load-lowering valve LSV during lowering.

FIG. 3 shows a further exemplary embodiment in which the additionalloads Z1, Z2 are supplied via a constant flow system KSS. On the liftingdrive side, the circuit is identical to that in FIG. 1. The furtherloads Z1, Z2 are connected in series via the valves V1, V2 and aresupplied by a volume flow which is independent of the level of the loadpressure. Although such a constant flow system does not provide thepossibility of adapting the pressure and volume flow to theinstantaneous requirements of the loads, it is of significantly simplerdesign than the load-sensing system illustrated in FIGS. 1 and 2. Whathas been stated about the method of functioning with respect to FIG. 1applies in the same way. Likewise, the circuit for the lowering modewith a low load can be extended with a flow-regulating valve asdescribed with respect to FIG. 2.

The proposed circuit with a motor/double pump assembly therefore permitsefficient energy recovery from the potential energy of a liftingcylinder with simultaneous operation of further loads, while compared toconventional circuits the expenditure on pumps and on the valvetechnology is reduced.

1. Hydraulic circuit arrangement having a first pump (P1) which isdriven by an electric motor (M/G) and is assigned to a lifting cylinder(HZ), wherein, in order to recover energy, the electric motor (M/G) canbe operated as a generator and the first pump (P1) can be operated as ahydraulic motor in order to recover potential energy of the liftingcylinder (HZ), and having a second pump (P2) for supplying at least onefurther load, characterized in that the electric motor (M/G), the firstpump (P1) and the second pump (P2) are embodied as a motor/double pumpassembly (MDP), wherein in the lowering mode of the lifting cylinder(HZ) the first pump (P1) acts as a hydraulic motor with reversal of therotational direction while the second pump (P2) always operates as apump with a constant delivery direction, and wherein arranged betweenthe first pump (P1) and the lifting cylinder (HZ) is a priority valve(PRV) which, depending on requirements, distributes the volume flowwhich flows in from the first pump (P1) to the lifting cylinder (HZ) onthe one hand, and to the at least one further load on the other, and thevolume flow which flows in from the lifting cylinder (HZ) in thelowering mode accordingly to the at least one load and the first pump(P1).
 2. Hydraulic circuit arrangement according to claim 1, in whicharranged in the working line of the first pump (P1) between the priorityvalve (PRV) and the lifting cylinder (HZ) is an electricallycontrollable lifting valve (HV) which is partially opened duringsimultaneous operation of the lifting cylinder (HZ) and of the at leastone further load, and in its two limiting positions switches the workingline of the first pump (P1) to the closed and open positions, whereinthe lifting valve (HZ) opens fully if only when the lifting cylinder(HZ) is activated alone.
 3. Hydraulic circuit arrangement according toclaim 1, in which the priority valve (PRV) is controlled by the pressuredifference which occurs across the lifting valve (HV).
 4. Hydrauliccircuit arrangement according to claim 3, in which, in a first extremeposition of the priority valve (PRV), the first pump (P1) is connected,on the one hand, to the lifting valve (HV) via a throttle and, on theother hand, to the at least one further load.
 5. Hydraulic circuitarrangement according to claim 3, in which, in a second extreme positionof the priority valve (PRV), the working line which comes from the firstpump is switched to the open position and at the same time is connectedto the at least one further load.
 6. Hydraulic circuit arrangementaccording to claim 3, in which the priority valve (PRV) has a centralposition in which the working line of the first pump (P1) is switched tothe open position and the line to the at least one further load isclosed.
 7. Hydraulic circuit arrangement according to claim 1, in whichprovided in the line leading from the priority valve (PRV) to the atleast one further load is a non-return valve (RÜV) which opens whenthere is excess pressure in the working line leading from the first pump(P1) to the lifting cylinder (HZ) compared to the pressure generated bythe second pump (P2).
 8. Hydraulic circuit arrangement according toclaim 1, in which the at least one further load is supplied by aload-sensing system (LSS).
 9. Hydraulic circuit arrangement according toclaim 1, in which the at least one further load is supplied by aconstant flow system (KSS).
 10. Hydraulic circuit arrangement accordingto claim 1, in which an electrically controllable load-lowering valve(LSV) is provided, and in the lowering mode said load-lowering valve(LSV) diverts the volume flow from the lifting cylinder to the reservoir(T) without the recovery of energy.
 11. Hydraulic circuit arrangementaccording to claim 10, in which pressure sensors (D1, D2) for actuatingthe load-lowering valve (LSV) are respectively provided in the workinglines of the first and second pumps (P1, P2).